The first results from observations of the high mass X-ray binary LS 5039 using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009 June are presented .
Our results indicate variability that is consistent with the binary period , with the emission being modulated with a period of 3.903 \pm 0.005 days ; the first detection of this modulation at GeV energies .
The light curve is characterized by a broad peak around superior conjunction in agreement with inverse Compton scattering models .
The spectrum is represented by a power law with an exponential cutoff , yielding an overall flux ( 100 MeV – 300 GeV ) of 4.9 \pm 0.5 ( stat ) \pm 1.8 ( syst ) \times 10 ^ { -7 } photon cm ^ { -2 } s ^ { -1 } , with a cutoff at 2.1 \pm 0.3 ( stat ) \pm 1.1 ( syst ) GeV and photon index \Gamma = 1.9 \pm 0.1 ( stat ) \pm 0.3 ( syst ) .
The spectrum is observed to vary with orbital phase , specifically between inferior and superior conjunction .
We suggest that the presence of a cutoff in the spectrum may be indicative of magnetospheric emission similar to the emission seen in many pulsars by Fermi .